Corneal surgery is currently undergoing rapid evolution with improvements designed to minimize or eliminate astigmatism following penetrating keraplasty (corneal transplants), as well as to correct refractive error. Because the cornea is the most powerful refracting surface of the eye, numerous procedures have been devised to incise, lathe, freeze, burn and reset the cornea to alter its shape. Currently practiced keratorefractive surgical techniques include: cryorefractive techniques (keratomileusis, keratophakis, ipikeratophakia), radial keratotomy, thermal keratoplasty, corneal relaxing incisions and wedge resections.
When preparing the patient for any of these surgical techniques, it is essential to accurately measure the corneal curvature. Existing methods to measure corneal curvature include central keratometry and photokeratoscopy with central keratometry. However, with these methods the diameter of the cornea that can be accurately measured is limited. Recently, photokeratoscopy has been adapted to provide a topographic map of the cornea. However, existing keratometers are limited in two important regards. Firstly, these instruments are predicated on geometrical image forming principles and assume the corneal topography can be expressed in terms of zones of various spherical radii. This in turn involves assumptions as to the nature of the surface under test. With more strongly aspheric corneas or as larger areas on even the average or typical size cornea are considered, the measurement becomes extremely ambiguous. Secondly, primarily as a consequence of the above, but also because of optical engineering considerations, most instruments are limited in terms of the aperture of the cornea that can be measured. Existing instruments typically cover corneal diameters no greater than three millimeters.
It is recognized that optical interferometric techniques for non-invasive measurement of corneal topography provides a way of producing a contour map of the corneal surface directly. A suitable achromatic or white-light interferometer is disclosed in co-pending patent application, Ser. No. 07/364,165, filed Jun. 12, 1989 and of which this application constitutes a continuation-in-part. However, a disadvantage of prior art interferometric corneal mapping devices is the requirement for a scanning operation in order to have sufficient phase information for mapping the entire surface of the cornea. This means that the cornea must be stationary during the scanning process. If the scanning can be performed faster than typical eye movements, that difficulty can be overcome. However, a technique that does not require scanning and captures the topographical information in one short duration exposure would be preferable. Thus, there is an ongoing need for a real-time "snapshot" keratometer system for medical diagnosis and for preparation of a corneal contour for eye surgery, as well as for post-operative analysis of completed eye surgery.